1. Field of the Invention
The invention relates to methods and apparatuses for reproducing half-tone images for image data having tone components. More particularly, the invention is directed to an improvement of a method and apparatus for reproducing half-tone images by converting continuous-tone image data into a multi-valued level density code using a predetermined threshold or thresholds.
2. Description of the Related Art
Known as a conventional method of reproducing half-tone images is an error diffusing method by Floyd, in which a predetermined algorithm is used in, e.g., binary-coding continuous-tone image data of a target pixel (R. Floyd & L. Steinberg: "An Adaptive Algorithm for Spatial Grayscale," SID Symp., Digest of Papers, p. 36 (1975)). According to the above method, the influence of a difference error between the image data and the threshold is diffused into surrounding pixels to thereby improve the reproducibility of half-tone images by binary-coding (See Published Unexamined Japanese Patent Application Nos. 228372/1989 and 158877/1989).
By the way, when a user operates a full-color image output apparatus adopting such a half-tone image reproducing method to produce a hard copy, which is a final output, the produced color tone is often different from what has been expected due to the operating environment and secular changes.
For this reason, what has been called for is a function in which the user can adjust the color tone as desired by a simple operation. Conventional color tone adjusting methods include a method of changing the masking constant when the RGB color representing system is converted to the YMC color representing system, and a method of adjusting the density of each color by changing the developing bias voltage in the case of electrophotography, or by changing energy to be applied in the case of ink jet recording systems or thermal recording systems.
However, in the method of changing the masking constant out of the conventional color tone adjusting technology, a number of masking constants, as many as 9 (at least), must usually be changed. When performing the masking operation by using a look-up table or the like, complicated operations including the operation of rewriting enormous amounts of table contents are entailed.
Also, color image output apparatuses adopting the electrophotographic system, the ink jet recording system, the thermal recording system, or the like have encountered the problem that there is a limitation in fine-tuning the density at the output hardware, which problem has not yet been solved.
These technical problems are similarly encountered not only by half-tone full-color image output apparatuses but also by half-tone single-color image output apparatuses.